1. Field of the Invention
The present invention relates to a control device for an internal combustion engine, and in particular, to a control device for an internal combustion engine employing PCCI (Premixed Charge Compression Ignition) combustion, capable of realizing high-stability operation of the engine throughout a large operation area of the engine.
2. Description of the Related Art
In the conventional diffusive combustion method widely employed for diesel engines, combustion of fuel in each combustion chamber is attained by causing self ignition of the fuel, by compressing air introduced into the combustion chamber and injecting the fuel into the compressed air. Although such a diesel engine exhibits higher thermal efficiency compared to gasoline engines, the air-fuel mixture density in the combustion chamber tends to be highly nonuniform (due to uneven distribution of the injected (sprayed) fuel in the combustion chamber) and the combustion in the combustion chamber is likely to separate into a combustion region of spraying flames and an air region, resulting in highly uneven combustion temperature distribution in the combustion chamber. This means that a region with excessively high fuel concentration, a region at excessively high temperature, etc. (i.e., a region in which PM (Particulate Matter) tends to be generated, a region in which NOx tends to be generated, etc.) coexist in the combustion chamber. Therefore, it is difficult to reduce both NOx and PM at the same time, and this leads to high emission of NOx and PM and a heavy load on the exhaust post-processing system.
In consideration of the above problems, improvement of combustion by means of premixed combustion has been discussed in recent years with increasing attention in order to drastically reduce the emission of NOx and PM while securing low fuel consumption (high mileage). For the combustion improvement of diesel engines by means of premixed combustion, two combustion methods, HCCI (Homogeneous Charge Compression Ignition) combustion and PCCI (Premixed Charge Compression Ignition) combustion, are being considered.
The HCCI combustion is based on the idea of uniformly mixing air and fuel in the intake pipe (as in gasoline engines), introducing the uniform (homogeneous) air-fuel mixture into the combustion chamber, and attaining combustion by causing the self ignition by compression.
In the HCCI combustion method, the air-fuel mixture is introduced into the combustion chamber in the intake stroke after injecting the fuel into the intake port and preparing a uniform (homogeneous) air-fuel mixture. With the sufficient mixing of fuel and air, the whole air-fuel mixture is kept in a lean condition, by which the occurrence of air-fuel ratios close to the theoretical mixture ratio (generally occurring in diesel engines) is prevented and low combustion temperature is achieved, leading to substantially no generation/emission of NOx. However, combustion of all the fuel in the combustion chamber is difficult and fuel adhering to the inner surface of the cylinder is discharged without combustion, resulting in high emission of HO (hydrocarbon). Further, the thermal efficiency tends to be relatively low in HCCI combustion since the control of ignition timing is difficult and is dependent on the temperature in the cylinder (combustion chamber) being compressed.
On the other hand, PCCI combustion is based on the idea of executing the premixing (of fuel and air) by injecting the fuel into the cylinder in the compression stroke similarly to ordinary diesel engines. While the fuel injection timing in conventional diesel engines is approximately BTDC (Before Top Dead Center) 10 degrees or later in terms of the crank angle, the PCCI combustion method (focusing on increasing the turbulent mixing speed rather than increasing the air-fuel mixing time) aims to start the ignition after completion of the fuel injection, while also delaying the fuel injection timing compared to conventional combustion methods.
With ignition timing delayed by 10-15 degrees compared to conventional diesel engines, the compression ignition is carried out to the premixed air-fuel mixture that has already been attenuated to some extent, by which combustion at low temperature, with no NOx generation and with less PM generation is realized.
FIG. 20 is a graph showing a PM generation area and a NOx generation area in regard to local temperature and local equivalence ratio. As shown in FIG. 20, PM is generated in a part with high local equivalence ratio (i.e., with excessively high fuel concentration) due to lack of oxygen, while NOx is generated in a part with low local equivalence ratio and high local temperature.
As indicated with the broken line Y, the conventional diffusive combustion (executing the compression ignition to unevenly-distributed sprayed fuel) occurs in a large area extending across both the PM generation area and the NOx generation area In contrast, as indicated with the solid line X, the PCCI combustion (executing the compression ignition to properly attenuated air-fuel mixture and thereby reducing the region with excessively high fuel concentration and the region at excessively high temperature compared to the conventional diffusive combustion) is capable of decreasing the generation of PM and NOx.
About PCCI combustion engines of this kind, a variety of studies has been conducted in these years. For example, Japanese Patent Application Publication Nos. 2003-532828 and 2003-532829 (hereinafter referred to as “patent documents #1 and #2”), aiming to further improve emission quality of exhaust gas by increasing combustion efficiency and reducing unburned HC and CO (carbon monoxide), have disclosed an engine equipped with a mixing device for supplying premixed air-fuel mixture by mixing a first fuel with the intake air and a direct fuel injector for injecting a second fuel directly into the combustion chamber. In this engine, the direct injection of the second fuel is carried out after the premixed air-fuel mixture has ignited in the combustion chamber. As the first and second fuels, combinations of various kinds of fuels (natural gas, gasoline, light oil, naphtha, propane gas, etc.) are disclosed in the patent document #1 and #2.
However, in an internal combustion engine employing the conventional PCCI combustion method described in the patent document #1 and #2, the combustion progresses too rapidly in a middle/high-load operation area of the engine, causing loud noise and strong vibration.
While it is possible to alleviate these problems to some extent by using fuel with a high cetane number or by delaying the fuel injection timing compared to the standard injection timing of the internal combustion engine, the method delaying the fuel injection timing is also known to seriously deteriorate fuel consumption (mileage) and stability of ignition and combustion. Since internal combustion engines employing the PCCI combustion have the above problems in the middle/high-load operation area, the PCCI combustion method in the present state can be only used in a low-load operation area.